1. Field of the Invention
This invention relates to a novel superconducting material having an RBa.sub.2 Cu.sub.4 O.sub.8 (R=a rare earth element) crystallographic phase.
2. Description of the Prior Art
A superconducting material having a composition RBa.sub.2 Cu.sub.3 O.sub.7 (R=a rare earth element) is known to have a high superconducting transition temperature Tc higher than the boiling point (77K) of liquid nitrogen (Appl. Phys. Lett. 51, 57(1987)). The oxygen content of this material varies according to conditions such as, temperature and oxygen partial pressure, under which the material has been sintered, so that the crystal phase thereof shifts between the tetragonal and orthorhombic systems. As a consequence of this phase change, Tc of the superconducting material considerably varies in the range of from 90K to 0K (insulation). Thus, it would be possible to control the Tc of the superconducting material to a desired point by controlling the oxygen content thereof. However, the superconducting material with a controlled Tc cannot be suitably used for the preparation of superconductor articles. Namely, when such a superconducting material is used as a raw material for the preparation of, for example, a wiring rod by a silver-sheathed wiring method in which a silver pipe filled with powder of the superconducting material is subjected to cold drawing, followed by sintering at 800.degree.-900.degree. C., the superconductivity deteriorates due to loss of its oxygen during the sintering stage. It is, therefore, extremely difficult to obtain a superconductor having a desired Tc using RBa.sub.2 Cu.sub.3 O.sub.7.
There is proposed a high Tc superconducting material composed of RBa.sub.2 Cu.sub.4 O.sub.8 (R=Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm) and having a three-layered pervoskite crystal structure with double CuO one-dimensional chains (Phys. Rev. B. 39, 7347(1989)). This material does not encounter increase or decrease in its oxygen content at temperatures up to about 850.degree. C. and, thus, is stable in such an environment. However, no means have been proposed for controlling Tc to a desired point. Another problem of this superconducting material is raised when used in actual industrial applications and relates to the fact that it is necessary to use, as a raw material for the production thereof, Ba(NO.sub.3).sub.2 which is expensive and poisonous.